see also subpage Dehydration
Deserts are located where vegetation cover is exceedingly sparse. This corresponds to the 'hyperarid' regions of the earth, where rainfall is exceedingly rare and infrequent.
Deserts are part of a wider classification of regions that, on an average annual basis, have a moisture deficit (i.e. they can potentially lose more than is received). These areas are collectively called 'drylands' and extend over almost a third of the earth's land surface. Because desert is a vague term, the use of 'dryland', and its subdivisions of hyper arid, arid, semiarid and dry-subhumid, is preferred in some contexts, and is approved by the United Nations.
A satellite image of the Sahara, the world's largest hot desert and second largest desert after Antarctica.
Deserts take up one-third of the Earth's land surface. They usually have a large diurnal and seasonal temperature range, with high daytime temperatures (in summer up to 45 °C or 113 °F), and low night-time temperatures (in winter down to 0 °C; 32 °F) due to extremely low humidity. Water acts to trap infrared radiation from both the sun and the ground, and dry desert air is incapable of blocking sunlight during the day or trapping heat during the night. Thus during daylight most of the sun's heat reaches the ground. As soon as the sun sets, the desert cools quickly by radiating its heat into space. Urban areas in deserts lack large (more than 25 °F/14 °C) daily temperature ranges, partially due to the urban heat island effect.
Many deserts are formed by rain shadows, mountains blocking the path of precipitation to the desert. Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony surfaces called hamada surfaces compose a minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation.
The snow surface at Dome C Station in Antarctica is a representative of the majority of the continent's surface.
Bottomlands may be salt-covered flats. Eolian processes are major factors in shaping desert landscapes. Cold deserts (also known as polar deserts) have similar features but the main form of precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98 percent thick continental ice sheet and 2 percent barren rock). Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that evaporate even ice.
The largest hot desert is the Sahara.
Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Due to extreme and consistent dryness, some deserts are ideal places for natural preservation of artifacts and fossils.
English desert and its Romance cognates (including Italian deserto, French désert and Spanish desierto) all come from the Latin desertum, which means "an unpopulated place". This in turn is derived from the Egyptian word dSr.t, which literally means "red land" and refers to the desert. The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies, and thus the use of the word desert can cause confusion. In English prior to the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation)—and a desert may be quite heavily populated, with millions of inhabitants. Phrases such as "desert island" and "Great American Desert" in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population. But the connotation of a hot, parched, sandy place often influences today's popular interpretation of those phrases.
The Thar Desert near Jaisalmer, India
High desert in Eastern Oregon, United States
The Agasthiyamalai hills cut off Tirunelveli (India) from the monsoons, creating a rainshadow region
In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters (10 in) of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters (10-20 in). Arid and extremely arid lands are deserts, and semiarid grasslands are generally referred to as steppes.
Measurement of rainfall alone can't provide an accurate definition of what a desert is because being arid also depends on evaporation which depends in part on temperature. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters (10 in) of precipitation per year, but is not generally recognized as a desert region.
So "potential evapotranspiration" supplements measurement of rainfall in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P-PE+/-S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation, coupled with the evaporative loss of water through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona receives about 300 millimeters, (12 in), of rain per year, however about 2500 millimeters, (100 in), of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in other regions such as Alaska are much lower.
There are different forms of deserts. Cold deserts can be covered in snow or ice; frozen water unavailable to plant life. These are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless.
Most non-polar deserts are hot in the day and chilly at night (for the latitude) because of the lack of the moderating effect of water. In some parts of the world, deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture.
Deserts are also classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in non-arid environments are paleodeserts.
Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalaya especially in Ladakh region of Jammu and Kashmir (India), in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (10,000 ft) and the thermal regime can be hemiboreal. These places owe their profound aridity (the average annual precipitation is often less than 40 mm/1.5in) to being very far from the nearest available sources of moisture. Montane deserts are normally cold.
Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction the wind is going. As the air moves over the mountains, it cools and moisture condenses, causing precipitation on the windward side. Moisture almost never reaches the leeward side of the mountain, resulting in a desert. When that air reaches the leeward side, the air is dry, because it has already lost the majority of its moisture. The air then warms, expands, and blows across the desert. The warm, desiccated air takes with it any remaining small amounts of moisture in the desert.
Deserts have a reputation for supporting very little life, but in reality deserts often have high biodiversity, including animals that remain hidden during daylight hours to control body temperature or to limit moisture needs. Some fauna includes the kangaroo rat, coyote, jack rabbit, and many lizards. Some flora includes shrubs, Prickly Pears, and the Brittle bush.
Flora of Baja California Desert, Cataviña region, Mexico
Most desert plants are drought- or salt-tolerant, such as xerophytes. Some store water in their leaves, roots, and stems. Other desert plants have long taproots that penetrate to the water table if present, or have adapted to the weather by having wide-spreading roots to absorb water from a greater area of the ground. Another adaptation is the development of small, spiny leaves which shed less moisture than deciduous leaves with greater surface areas. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Even small fungi and microscopic plant organisms found on the soil surface (so-called cryptobiotic soil) can be a vital link in preventing erosion and providing support for other living organisms.
Saguaros in the Sonoran Desert of Arizona
Deserts typically have a plant cover that is sparse but enormously diverse. The giant saguaro cacti of the Sonoran Desert provide nests for desert birds and serve as "trees" of the desert. Saguaro grow slowly but may live up to 200 years. When 9 years old, they are about 15 centimeters (6 in) high. After about 75 years, the cacti develop their first branches. When fully grown, saguaro cacti are 15 meters tall and weigh as much as 10 tons. They dot the Sonoran and reinforce the general impression of deserts as cactus-rich land.
are often thought of as characteristic desert plants, other types of
plants have adapted well to the arid environment. They include the pea
and sunflower families. Cold deserts have grasses and shrubs as
Out ìn the desert, it's all to easy to become dehydrated. Unless you
have spent any amount of time ìn such dry places, you mìght not be
aware that the arid location ìs leeching water from your body until
it's too late. Part of what defines the desert ìs the fact that there
ìs no water. Most of the plants and animals that do live there have
already adapted to the harsh environment, but no matter how well humans
have adapted over the centuries, they are stìll not as well prepared.
The need for water ìs second only to that of air - you can't go for
very long without ìt and survive.
Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters (1.7 in) of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous.
Though little rain falls in deserts, deserts receive runoff from ephemeral, or short-lived, streams fed considerable quantities of sediment for a day or two. Although most deserts are in basins with closed or interior drainage, a few deserts are crossed by 'exotic' rivers that derive their water from outside the desert. Such rivers infiltrate soils and evaporate large amounts of water on their journeys through the deserts, but their volumes are such that they maintain their continuity. The Nile River, the Colorado River, and the Yellow River are exotic rivers that flow through deserts to deliver their sediments to the sea. Deserts may also have underground springs, rivers, or reservoirs that lie close to the surface, or deep underground. Plants that have not completely adapted to sporadic rainfalls in a desert environment may tap into underground water sources that do not exceed the reach of their root systems.
Lakes form where rainfall or meltwater in interior drainage basins is sufficient. Desert lakes are generally shallow, temporary, and salty. Because these lakes are shallow and have a low bottom gradient, wind stress may cause the lake waters to move over many square kilometers. When small lakes dry up, they leave a salt crust or hardpan. The flat area of clay, silt, or sand encrusted with salt that forms is known as a playa. There are more than a hundred playas in North American deserts. Most are relics of large lakes that existed during the last ice age about 12,000 years ago. Lake Bonneville was a 52,000 kilometers² (20,000 mi²) lake almost 300 meters (1000 ft) deep in Utah, Nevada, and Idaho during the Ice Age. Today the remnants of Lake Bonneville include Utah's Great Salt Lake, Utah Lake, and Sevier Lake. Because playas are arid landforms from a wetter past, they contain useful clues to climatic change.
When the occasional precipitation does occur, it erodes the desert rocks quickly and powerfully.Winds are the other factor that erodes deserts—they are slow yet constant.
The flat terrains of hardpans and playas make them excellent racetracks and natural runways for airplanes and spacecraft. Ground-vehicle speed records are commonly established on Bonneville Speedway, a racetrack on the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at Edwards Air Force Base in California.How to Make Water in the Desert
The desert is hot and dehydration can set in quickly. If you are stuck or stranded in the desert, you can use the sun to generate a sufficient amount of water for yourself. You will probably, however, lose more water perspiring while building the still than it will generate for you in the end. This article discusses the concept of a "Solar 'still", or distillery. It can be used to collect water from soil in any climate, given enough sunlight and existent soil moisture.